Sulfuric acid is one of the most widely used chemicals in industrial chemistry. Though not often evident in the final product, sulfuric acid plays a significant role in the production of many chemical products. Most sulfuric acid is discharged from industrial production either as a waste sulfate or in the form of sulfuric acid. Either waste can be discharged in varying degrees of dilution and contamination. The treatment and disposal of spent sulfuric acid from industrial waste streams has long been a problem in many industries.
Sulfate route titanium dioxide industries use concentrated sulfuric acid to digest ilmenite ore, and after precipitation of titanium dioxide, the spent acid needs to be removed. The recycling or disposal of the acid is one of the most difficult hurdles facing the industry. In chloride route titanium dioxide manufacture, sulfuric acid is used to clean the outlet gas from the oxidizer, contains un-reacted titanium chlorides and trace amounts of iron chlorides. This acid is replaced by fresh acid once the specific gravity exceeds a particular limit.
Neutralization is the most popular method for treating waste sulfuric acid solutions. To neutralize sulfuric acid, a variety of bases are added to a sulfuric acid wastewater stream until the stream has been neutralized. A considerable drawback to the total neutralization process is the huge quantity of solid waste generated, which generally requires landfill disposal. Increasingly, higher disposal costs, numerous environmental issues connected with traditional neutralization and landfill practices, and the diminishing amount of landfill space, have led to the need to develop alternative methods to re-use sulfuric acid rather than neutralize it.
Although various alternative methods are available for sulfuric acid treatment and disposal, most of them are very specific, highly dependent on particular impurities present, and dependent on the degree of acid dilution. For example, reverse osmosis has been used to treat or dispose of sulfuric acid. Reverse osmosis forces waste sulfuric acid through costly filtration systems until the acid content of the stream is reduced to a level where the remaining stream can be disposed of by conventional means. This process requires an expensive filtration system that is generally difficult to build and maintain. Moreover, current reverse osmosis filtration systems are only effective for treating small volume streams.
Evaporation is another disposal and treatment method for waste sulfuric acid. Dissipating or removing water from an aqueous sulfuric acid solution, however, requires significant energy input and therefore carries a high cost. The removal of impurities before evaporation presents a further complication. Incineration may also be used to dispose of waste sulfuric acid, but like evaporation, incineration may lead to the creation of acid rain, rendering the process environmentally unacceptable.
U.S. Pat. No. 5,527,985 describes a process for recycling waste sulfuric acid by photo-decomposition of waste acid that is contaminated by organic compounds.
High temperature recycling of highly contaminated waste sulfuric acid by decomposition in water, SO2, and oxygen is described in Winnacker, Kichler; Chemische Technologie; Vol. 2; Page 1; 4th edition. This type of recycling is also performed when the acid is contaminated with organic compounds.
EP 132820 B1 describes the possibility of evaporating dilute acid, optionally extracting metal sulfates, and reacting the remaining metal sulfates with CaO, Ca(OH)2 and/or CaCO3 to form gypsum and sparingly soluble metal compounds. This method, however, is not very economical, in particular because of the energy needed to concentrate the dilute acid. The processing of other spent sulfuric acids that contain heavy metals is generally also costly and uneconomical.
WO 1998/043716 A2 describes an aqueous sulfuric acid purification process that uses freeze concentration involving cooling the aqueous sulfuric acid solution to a temperature at or near its freezing point and separating the resulting acid-rich region from the acid-poor region. U.S. Pat. No. 5,394,706 also describes a freeze concentration method. The acid rich-region is separated from acid-poor region either by centrifuging or by using a density separation column. This method, however, fails when impurities are dissolved uniformly because the impurities are also frozen along with the acid. Moreover, this method cannot purify the acid to minute levels of impurities. The energy efficiency of this method is also of concern.
U.S. Pat. No. 7,537,749 B2 describes a method for processing heavy metal-laden spent sulfuric acid. Iron-laden spent sulfuric acid or iron-laden sulfuric materials are reacted with a material that contains iron chloride and optionally other metal chlorides, producing iron (II) sulfate. The spent sulfuric acid can come from a sulfate route titanium dioxide production.
Both reuse and disposal of spent sulfuric acid has become a nuisance for industries that use acid either as a reactant or as an auxiliary chemical. In most cases, the regeneration of spent sulfuric acid is more expensive than the production of fresh acid from raw materials. Given the limitations in current disposal methods, there exists a need for a cost effective and environmentally prudent method to treat and/or dispose of waste sulfuric acid, particularly sulfuric acid waste resulting from the chloride route for manufacturing titanium dioxide.